CONCRETIONS are concentrations of inorganic sedimentary material within
other sediments. Minerals that commonly form concretions are silica (in
the form of opal, chert, chalcedony, and quartz), calcite, siderite,
pyrite, marcasite, and limonite.
Concretions may form either as the sediment around them is forming or
after the sediment around them has hardened. They may be formed when
water containing dissolved minerals seeps through the sediment or rock
and leaves a concentration of mineral matter in a cavity or around a
central particle (nucleus) such as the remains of a plant or animal.
Portions of rock may also become firmly cemented by such mineral matter.
Concretions range in size from minute particles to objects several feet
in diameter. Shapes range from spheres to tubes. Many are globular or
lumpy-surfaced, some are smooth. Because concretions generally are
harder than the surrounding rock in which they have formed, they do not
weather away as readily and may remain after the surrounding material
has been eroded.
Concentrations of calcite are found in loess deposits. They may look
like bizarre, knobby figurines, and the Germans called them loess
kindchen (little children of the loess).
Ironstone concretions, especially common in many Illinois shales, are
formed by a local concentration of the mineral siderite (iron carbonate)
in the rock. The concretions found in weathered outcrops commonly are
partly or entirely weathered to limonite. Some ironstone concretions
grow together into odd shapes. Mazon Creek ironstone concretions of
northeastern Illinois, world famous for their fossils, are sideritic.
The concretions are commonly covered with limonite, the result of
oxidation.
Limonite concretions, generally with a high content of clay, silt, or
sand, occur in loess, shale, and sandstone.
Concretions of chert and other forms of silica are common in limestones.
In many places, because of their greater resistance to weathering,
lenses and nodules of chert protrude from the beds.
Pyrite or marcasite occur as concretions or concretion-like masses in
some coal beds and in the black shales, sometimes popularly called
“slates,” above coal beds. Some other Pennsylvanian clays and shales
also contain concretions or coarsely crystalline aggregates of these
minerals.